I picked up what I consider a rather valuable chunk from two master club players concerning the movement of your a-pawn: If you’ve got the initiative as White on the kingside and you have to respond to Black trying to muster counterplay on the queenside, you respond to his queenside expansion by playing a4! Whereas if you are Black waiting to create some counterplay against White while defending against his initiative on the queenside, you respond to a queenside expansion with the calm a6. Long story short: If you’re on the attacking side (having the initiative), move your pawns aggressively to claim more space; if you’re defending (not having the initiative), move your pawns defensively to protect the space in your terrain.